Scroll-type devices are well known in the field of vacuum pumps and compressors. In a scroll device, a movable spiral blade orbits with respect to a fixed spiral blade within a housing. The movable spiral blade is connected to an eccentric drive mechanism. The configuration of the scroll blades and their relative motion traps one or more volumes or “pockets” of a gas between the blades and moves the gas through the device. Most applications apply rotary power to pump the gas through the device. Other applications include expanders, which operate in reverse from compressors and extract power from the expansion of a pressurized gas.
A scroll pump includes stationary and orbiting scroll elements, and a drive mechanism. The stationary and orbiting scroll elements each include a scroll plate and a spiral scroll blade extending from the scroll plate. The scroll blades are intermeshed together to define interblade pockets. The drive mechanism produces orbiting motion of the orbiting scroll element relative to the stationary scroll element so as to cause the interblade pockets to move toward the pump outlet.
For proper function of the scroll pump, it is necessary to maintain a fixed angular relation, or synchronization, between the two scroll elements. Scroll pumps typically utilize one or more devices for synchronizing the intermeshed scroll blades. Each synchronizing device is coupled, directly or indirectly, between the stationary and orbiting scroll elements and is required to permit orbiting movement while preventing relative rotation of the scroll elements. In one prior art approach, disclosed in U.S. Pat. No. 801,182 issued Oct. 3, 1905, three crank mechanisms are connected between the orbiting and stationary scroll elements.
Oil-lubricated scroll devices are widely used as refrigerant compressors. Oil-lubricated scroll pumps have not been widely adopted for use as vacuum pumps, mainly because the cost of manufacturing a scroll pump is significantly higher than a comparably-sized, oil-lubricated vane pump. In cases where oil contamination is unacceptable, dry scroll pumps are used. Normally these pumps contain multiple rolling element bearings which require lubrication. One approach to lubrication is to use a low-vapor-pressure synthetic grease. However, some degree of contamination can still occur when the bearings are located within the vacuum space of the pump. In addition, the lubricating performance of such greases is generally inferior, and their cost higher, than equivalent petroleum greases.
Accordingly, methods have been devised to isolate the bearings from the pumping mechanism while still permitting the relative orbital motion of the fixed and moving scroll elements. U.S. Pat. No. 5,951,268, issued Sep. 14, 1999, describes the use of a flexible metal bellows for isolation of the running gear of a scroll pump, also relying on the bellows for synchronization of the scroll elements. The torsional load on the bellows due to its function in synchronization poses a risk of failure due to metal fatigue. U.S. Pat. No. 7,261,528, issued Aug. 28, 2007 to assignee of the present invention, describes the use of a rectangular flexible metal element for synchronization as well as to take axial loads, while using a bellows, rotatably mounted, for isolation.
Prior art use of tubular bellows for isolation requires that the bellows be of sufficient length to reduce the stresses in the bellows material below the fatigue life limit for the material. Increased bellows length increases the length of the pump, which may be unacceptable in many applications. Consequently, improved methods of isolating the running gear of a scroll pump from the vacuum space are needed.